1. Field Of The Invention
The invention relates to a hydrophobically coated abrasive grain based on corundum or silicon carbide, to its use in synthetic resin bonded abrasives, and to a process for producing it.
2. Background Art
The performance efficiency of elastically or rigidly bonded abrasives is determined not only by the hardness and toughness of the abrasive grain, but to a considerable extent also by the strength of the bonding. In synthetic resin bonded abrasives the essential water resistance of the bonding represents a considerable technical problem for wet grinding. The most frequently used abrasive grains based on corundum or silicon carbide have good water wettability without special preliminary treatment, which makes it possible for water to penetrate between the grain and the bonding agent and which entails a weakening of the bonding after storage or use under water.
Therefore, quite a few attempts have been made to make abrasive grain hydrophobic, i.e., water repellant, to prevent or at least reduce such decline in the strength of the bonding under the effect of water. There has been a partial success through treatment with silanes, such as, (3-aminopropyl)triethoxy-silane, as described in manufacturer's publication SF-1159 of Union Carbide Corporation (Silicons Division). However, it has been shown that the improvement in the characteristics of the abrasive grain that can thus be achieved is not lasting, but rather disappears again after a few months of storage. The abrasive grain treated in this manner, therefore, had to be further processed as quickly as possible, which had the result that the treatment was suitably undertaken only at the place of manufacture of the abrasives, and not in large lots at the place of manufacture of the abrasive grain, which in itself would be more economical. Further, the process itself is not as free of problems as it first seems. Fine grain sizes (e.g., F120 to F1200) tend toward agglomerate formation. The conditions for the necessary drying with the silane solution after the wetting of the abrasive grain must be maintained exactly to prevent hydrolysis or thermal decomposition, and finally silane is by no means toxicologically harmless, which leads to environmental and safety problems at the places of manufacture of the abrasives which are often not equipped to handle dangerous materials.